If the final launch of the space shuttle Endeavour goes ahead as planned next week, it will be carrying an unusual cargo: baby squid.

This is not because the astronauts want a change in their menu: the squid could help us understand how "good" bacteria behave in the microgravity of space. As Jamie Foster of the University of Florida in Gainesville, who is running the experiment, puts it: "Do good bacteria go bad?"

This is a classic example of mutualism: the two species cooperate and each benefits. Humans have similar relationships with microbes, which help shape our immune and digestive systems, but thousands of species are involved with us rather than just one. "Humans are way too complex," Foster says.

Foster's experiment is simple. Newly hatched squid that have not yet encountered their bacterial partners will go up to orbit in tubes of seawater. Fourteen hours after launch, an astronaut will add the bacteria and give them 28 hours to colonise the squid. Then the squid will be killed and fixed solid, and brought back to Earth for examination.

Foster has some preliminary results from Earth-bound experiments that simulated microgravity and appeared to show problems with the uptake of bacteria by squid. If the shuttle study shows the same result, it would suggest that astronauts' relationships with their own microbes might also be affected in space. "We want to make sure the astronauts are healthy," she says.

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